Optical systems are currently the subject of a great deal of attention in telecommunications primarily due to their enormous information-handling capacity. A typical system includes, at the transmitter end, a source of light, such as a laser, a modulator for impressing information onto the light signal, and one or more optical amplifiers for amplifying the optical signal. The signal is usually transmitted by means of an optical fiber. At the receiver end, typically a photodetector such as a PIN diode or avalanche photodiode (APD) may be employed to convert the optical signal to an electrical signal. Of course, several other components, such as optical switches, circulators, and isolators may be employed.
For purposes of economy and size it is desirable to integrate as many devices as possible on a single substrate. For example, it is known to integrate a laser and modulator into a single device generally known as an Electroabsorption Modulated Laser (EML). It is also known to integrate a Distributed Bragg Reflection (DBR) laser, a modulator, a Semiconductor Optical Amplifier (SOA) and a monitor onto a single substrate. Normally, when integrating active device components (i.e., devices which provide optical gain), the devices are formed by Selective Area Growth (SAG) with modifications to the composition, thickness, or number of Quantum Well layers in the active regions of the various devices. In the case of two active devices, it is difficult to optimize the device characteristics and the butt joint which couples the two devices together. In the case of three or more devices, the problem of optimization becomes especially difficult.
It has been proposed to couple active devices in an integrated structure using a passive waveguide. (See, e.g., U.S. Pat. No. 5,134,671 issued to Koren et al, and U.S. Pat. No. 5,029,297 issued to Halemane, et al.) However, such devices provide coupling between waveguide and device in a vertical direction which can also be difficult to make since the waveguide needs to be formed in the same growth process as the active devices. It is generally more advantageous to provide a butt coupling between devices (i.e., the light coupling is done in a horizontal direction) so that the coupling is optimized independently from the active devices.
It is desirable, therefore, to provide an integrated optoelectronic device with at least two active components where device characteristics and butt joint coupling may be optimized.